Retroviruses form a sub-group of RNA viruses which, in order to replicate, must first "reverse transcribe" the RNA of their genome into DNA ("transcription" conventionally describes the synthesis of RNA from DNA). Once in the form of DNA, the viral genome may be incorporated into the host cell genome, allowing it to take advantage of the host cell's transcription/translation machinery for the purposes of replication. Once incorporated, the viral DNA is virtually indistinguishable from the host's DNA and, in this state, the virus may persist for the life of the cell.
A species of retrovirus, the Human immunodeficiency virus (HIV) has been reproducibly isolated from patients with AIDS (acquired immunodeficiency syndrome) or with the symptoms that frequently precede AIDS. AIDS is an immunosuppressive or immunodestructive disease that predisposes subjects to fatal opportunistic infections. Characteristically, AIDS is associated with a progressive depletion of T-cells, especially the helper-inducer subset bearing the CD4 surface marker. HIV is cytopathic and appears to preferentially infect and destroy T-cells bearing the CD4 marker, and it is now generally recognized that HIV is the etiological agent of AIDS. Clinical conditions such as AIDS-related complex (ARC), progressive generalized lymphadenopathy (PGL), Karposi's sarcoma, thrombocytopenic purpura, AIDS-related neurological conditions, such as AIDS dementia complex, multiple sclerosis or tropical paraparesis, and also anti-HIV antibody-positive and HIV-positive conditions, including such conditions in asymptomatic patients, are also conditions which may be treated by appropriate anti-viral therapy.
Another RNA virus which has been recognized as the causative agent of an increasingly serious international health problem is the non-A, non-B hepatitis virus. At least 80% of cases of chronic post-transfusional non-A, non-B hepatitis have been shown to be due to the virus now identified as hepatitis C and this virus probably accounts for virtually all cases of post-transfusional hepatitis in clinical settings where blood products are screened for hepatitis B. Whereas approximately half of the cases of acute hepatitis C infection resolve spontaneously over a period of months, the remainder become chronic and in many if not all such cases chronic active hepatitis ensues with the potential for cirrhosis and hepatocellular carcinoma. The structure of the hepatitis C virus genome has been elucidated and the virus has been characterized as a single stranded RNA virus with similarities to flaviviruses.
Hepatitis B virus (HBV) is a small DNA containing virus which infects humans. It is a member of the class of closely related viruses known as the hepadnaviruses, each member of which selectively infects either mammalian or avian hosts, such as the woodchuck and the duck. Recent insights into the mechanism of replication of the hepadnavirus genome indicate the importance of reverse transcription of an RNA intermediate, suggesting that the reverse transcriptase is a logical chemotherapeutic target. HBV is a viral pathogen of major world-wide importance. The virus is etiologically associated with primary hepatocellular carcinoma and is thought to cause 80% of the world's liver cancer. Clinical effects of infection with HBV range from headache, fever, malaise, nausea, vomiting, anorexia and abdominal pains. Replication of the virus is usually controlled by the immune response, with a course of recovery lasting weeks or months in humans, but infection may be more severe leading to persistent chronic liver disease outlined above.
U.S. Pat. No. 5,047,407 discloses (2R, cis)-4-amino-1-(2-hydroxymethyl-1,3-oxathiolan-5-yl)-(1H)-pyrimidin-2-one (Epivir.RTM., lamivudine) and its use in the treatment and prophylaxis of viral infections. Lamivudine has proven antiviral activity against HIV and other viruses such as HBV. Current liquid formulations of lamivudine used in the clinic contain disodium(ethylenedinitrilo)tetraacetate dihydrate (edetate disodium, EDTA) and 6% (v/v) ethanol. However, liquid formulations without ethanol or other sedatives and EDTA or other unnecessary anti-oxidants are considered advantageous, particularly for pediatric use and in renally or hepatically impaired adults.
The addition of alcohol and EDTA. is thought to be necessary in order to maintain preservative efficacy against bacteria, yeasts, and mold. EDTA, a chelating agent, has been shown to potentiate the activity of many antimicrobial agents by chelating Mg.sup.2+ and Ca.sup.2+ ions which are normally responsible for the stability of the cell wall of Gram-negative organisms. In a study of factors affecting preservative efficacy of lamivudine oral solution, Nguyen et. al. reported that preservative efficacy improved with increasing EDTA concentrations and with increasing pH from 4.5 to 7.5 (Nguyen, N-A. T., et. al., Drug Development and Industrial Pharmacy 21, 14, 1671-1682, 1995). The same study reported that the chemical stability of lamivudine increased with increasing pH from 4.5 to 7.5. Preservative efficacy was greatest at pH 7.5, but increasing the pH from 4.5 to 7.5 resulted in extensive degradation of preservatives such as esters of hydroxybenzoate (hereinafter referred to as parabens). All formulations were effective against bacteria and yeasts, but not against the mold, Aspergillus niger.
In a study evaluating the effects of alcohol concentration on preservative efficacy of lamivudine oral solution, Wells et. al. reported that the reduction or elimination of alcohol from lamivudine oral solutions resulted in unacceptable preservative efficacy (Wells et al., Pharmaceutical Research, 10(10), S171, 1993).
Lamivudine is currently formulated at pH 5.5 with 0.01% EDTA, 0.12% (w/v) methyl paraben, 0.015% propyl paraben, and 6% ethanol. In this formulation, EDTA functions both to maintain pH and preservative efficacy. At this concentration of parabens and pH, ethanol is needed in order to pass the Antimicrobial Preservatives Effectiveness (APE) test according to United States Pharmacopeia (USP) standards (United States Pharmacopeia 23, &lt;51&gt;, p. 1681, 1995), BP standards (Efficacy of Antimicrobial Preservation, Appendix XVI C, 1995), and PhEur standards (Efficacy of Antimicrobial Preservation, Chapter VIII.14, 1992). The pH was maintained at 5.5 in order to preserve the chemical stability of the parabens. We have surprisingly found that there is a sharp increase in preservative efficacy when lamivudine is formulated at pH&gt;5.5 (FIG. 1) and the concentrations of parabens are increased by 20-25% of the concentration of parabens in the ethanol-containing formulation.
We have found that the oral formulations of lamivudine according to the present invention surprisingly maintain preservative efficacy and chemical stability while eliminating ethanol and EDTA.
An object of the present invention is to provide pharmaceutical compositions comprising lamivudine and a preservative system that allows the elimination of ethanol and EDTA, while maintaining preservative efficacy.